USNA looks for academics, athletics and leadership. As a general rule, most students demonstrate leadership through ECAs (extra-curicular activities). It's not so much which activities you choose that is important but, rather, what you do in those activities.
It is better to be very active in a few activities than simply being a member of many. Thus, you may be active in student government and be class rep, school VP, etc. and be responsible for school projects. You may choose to be active in the Biology club and organize a major community project. You may be the VP of a student community club and develop and lead a project at your local senior citizens' center. You may be an Eagle Scout or a Gold Award winner. You may be in charge of your church's Sunday School program, developing schedules, teaching other teachers, etc. As you can see, there are many different ways to approach ECAs and leadership.
As to your particular situation, you're already a junior and (by your admission) have not previously been active in any ECAs. Thus, it's hard to show long-term commitment. Thus, I would focus on picking one or two ECAs that interest YOU and figuring out what you can do in those ECAs to make a difference. As noted above, that could be a school club, a religious activity, or a non-school activity. The key is that the mission of the organization is something that appeals to you.
Once you determine which activities in which you want to participate, figure out how you can make a difference. Is it a small organization where you can do things to increase membership? Is it an organization that has been rather inactive and you can do something to invigorate it? Can you think of a great project for the organization to take on and be in charge of making that project successful? Maybe you can even start up a new club at your school.
The possibilities are limitless. The bottom line is that it's NOT the activity you pick as much as what you do as part of that activity.